Titanium nitride and improved process for preparation of same



Patented Mar. 16, 1954 TITANIUM NITRIDE AND IMPROVED PROC- ESS FOR PREPARATION OF SAME Joseph J. Cacciotti, Perth Amboy, N. J., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey No Drawing. Application September 6, 1952, Serial No. 308,291

4 Claims.

This invention relates to the preparation of a refractory metal nitride and'more particularly to a substantially pure finely divided titanium nitride and an improved process for makin the same.

Many methods have been proposed for the preparation of titanium nitride including reacting titanium metal in a nitrogen atmosphere or reacting rutile ore or titanium dioxide with nitrogen in the presence of carbon at relatively high temperatures. In all Such methods the titanium nitride produced is either in massive form or in a coarse, granular, highly sintered state. Such products cannot be ground to a finely divided powder because of the hard and abrasive character of titanium nitride. Moreever, free carbon and/or graphite are usually present in the sintered products of the prior art While grinding procedures introduce additional impurities. Thus, there is no known method for producing finely divided titanium nitride of high purity in a commercially practicable manner.

An object, therefore, of the present invention is to provide a relatively inexpensive, finely divided titanium nitride of high purity.

A further object of the invention is to provide an improved method for producing relatively pure finely divided titanium nitride in an efiicient, economical manner. These and other objects will become apparent from the following more complete description of the instant invention.

The term finely divided" as used herein with reference to the size of the titanium nitride particles made by the process of thi invention shall be understood to mean primary particles in sizes from 0.01 to 1.0 micron.

The phrase primary particle size of less than 300 microns as used herein and in the following claims shall be understood to mean that the individual particles of titanium sulfide which make up the titanium sulfide powder are each less than 300 microns in size.

Broadly, the invention contemplates a method for the preparation of finely divided titanium nitride which comprises reacting titanium sulfide of predetermined particle size with anhydrous ammonia at an elevated temperatur and for a predetermined length of time to form substantially pure finely divided titanium nitride and gaseous by-products.

In carrying out the invention it was discovered that the successful production of finely divided substantially pure titanium nitride particles is dependent in large measure upon the nature of 2 the reactants and the length of time of the reaction.

As pointed out above, due to the hardness of titanium nitride, it is impossible, from a practical' standpoint, to grind coarse, granular lumps of titanium nitride to form a finely divided material. However, the instant invention embraces the discovery that Whereas titanium sulfide in large sizes, for example, in small lumps or large granules, will react with anhydrous ammonia only indifferently and, if at all, only to produce a composition comprisin a mixture of titanium nitride and sulfides; that by providing titanium sulfide in relatively small particle sizes such as, for example, less than 300 microns, the small primary particles of titanium sulfide will not only react with anhydrous ammonia to produce a substantially pure titanium nitride product but, quite unexpectedly, the primary particle siz of titanium nitride is many times smaller than the primary particle size of the titanium sulfide.

The term titanium sulfide as used in describing the process of this int ention shall be understood to include sulfides of titanium corresponding in composition to the formulas TiSz (titanium disulfide) TizSa (titanium sesquisulfide) and T15 (titanium monosulfide) and may be produced by any known method which will form a relatively soft powdery material readily susceptible to grinding or micropulverizing to produce primary particles within the size range mentioned above. By way of example, one method of producing titanium disulfide of high purity is by reacting titanium tetrachloride with anhydrous hydrogen sulfide at a temperature of about 500 0. accordin to the formula:

The titanium sulfide powder so produced has been analyzed as comprising about 42% titanium and about 56% sulfide, the remaining 2% being occluded hydrogen sulfide, oxygen, and metallic impurities. The reduced sulfides TizSs and TiS may be made according to the above reaction at temperatures of 1000 C. and 1400" C. respectively.

The size of the primary particles of titanium sulfide produced by th above reaction is generally from 200 to 800 microns but inasmuch as the material is relatively soft, it may be readily reduced in size by grinding or micropulverizing to a predetermined particle size of less than 300 microns, a satisfactory size for forming the finely divided titanium nitride material of this invention being from to 44 microns and preferably microns.

The anhydrous ammonia used in the thermal reaction with the titanium sulfide powder may be ammoniav gas or a hydrogen-nitrogen. mixture substantially. comparable in proportions to those of ammonia gas, and is preferably substantially chemically pure. Moreover, as hereinafter described, the anhydrous ammonia is adapted to be further purified and dehydrated prior toreaction with the titanium sulfide.

It is essential to high eificiencies of operation, good economy and a substantially pure finely divided product that the anhydrous ammonia be brought into intimate contact with the particles of powdered titanium sulfide at elevated temperatures and for a predetermined time interval. While this may be accomplished in anyone of several ways such as, for example, by passing ammonia-- gas through the titanium sulfide by static bed or fiuos olids techniques, highly satisfactory -..results have been obtained by carryingout the re- .action in a resistor type tube furnace." ll ith this ,type of reactorunit intimate contact between the titanium sulfide powder and the ammonia gas is .obtained by introducing the powdered titanium sulfide into a reaction tube formed preferably of alundum and closed at its opposite ends to prevent .air or. oxygen from entering the chamber during the reaction. A feed pipe is provided at oneend otthe reaction tube for connecting the latter to a source of anhydrous ammonia gas which is fed intcthe reaction tube to react with thetitanium sulfide therein; while an exhaust pipe is provided at the opposite end of the reaction .tube for conducting the gaseous by-products oi" the reaction into a suitable liquid-trap. To insure theremoval' of substantially all of the impurities .landmoisture from the ammonia gas, the latter .the rate of feed of the ammonia gas being de- ,pendent. upon the mol ratio of the ammonia and titaniumsulfide, and the time of the reaction. During the-time the ammonia gas is being fed iintdthe reaction tube, it is particularly desirable to agitate the powdered titanium sulfide by rotatthe reaction tube so as to insure intimate con- .tact between the powdered titanium sulfide and .the ammonia gasj Theoretically, if sufficient contact is provided between the titanium sulfide par ticles and the ammoniagas during the reaction, the amount of ammonia and titanimn sulfide used should approach stoichiometric proportions, that is, 2 mols ammonia gas to 1 mol titanium sulfide. However, in actual practice, it is desirable to use an amount of ammonia gas in excess of the stoichiometric amount both to insure complete reactionbetween the powdered titanium sulfide particles and the ammonia and to sweep out the gaseous by-products formed thereby so as to enhance vthe production of a substantially pure titanium nitride product. In this connection it has been observed that when the excess of ammonia. gas is relatively small such as, for example, substantially 2.5 mols of ammonia to 1 mol of titanium sulfide, the time required for complete t0 1.0Ii101oftitalliumsulflde, which corresponds o LA iii)

to excess ammonia, the time for complete reaction is from 2 to 4 hours, and the product is characterized by a high state of purity. Since it is desirable, commercially, to complete the reaction between the gaseous ammonia and the powdered titanium sulfide in as short a period of time as is practicable, it is preferable to use relatively large excesses of ammonia such as, for example, from 100 to 200%.

The reaction is carried out at elevated temperatures and in the absence of oxygen, the minimum temperature for insuring complete reaction between the gaseous ammonia and the titanium sulfide being about 800 C. At temperatures above about 1500 C. the product is partially sintered and a relativelylarge primary particle size is obtained. Within the temperature range of from 800 C. to 14200 C, the reaction between the ammonia and the titanium sulfide is substantially complete andthe titanium nitride product comprises a substantially pure finely divided material. While temperature changes within the preferred temperature range have little or no effect upon the completion of the reaction or the purity of the titanium nitride formed thereby, it has been noted that at the lower end of the temperature range such as, for example, at about 900 C., the titanium nitride is of minimum primary particle and is characterized by a violet color whereas at the upper end or" the temperature range such as, for example, 1275" C., the titanium nitride is or larger primary particle size and is characterized by a bronze color. Prior to introducing the ammonia gas into the reaction tube the latter is preferably flushed out with an inert gas to remove any oxygen whereupon the ammonia gas is introduced into the reaction tube, the reaction between the ammonia gas and the titanium sulfide goes forward at a substantially constant rate at temperatures within the range of from 800 C. to 1400" C. to produce a finely divided, substantially pure titanium nitride and the gaseous by-products. These gaseous by-products comprise essentially hydrogen sulfide, hydrogen and nitrogen and may be conducted oil from one end of the reaction tube through suitable liquid or other convenient traps. After sufilcient ammonia has been introduced into the tube to convert substantially all of the titanium sulfide values to titanium nitride, the tube is cooled and titanium nitride powder is removed. Depending upon the temperatures used during the reaction, the primary particles of finely divided titanium nitride may vary from 0.01 to 0.5 micron, the color of the smaller particles being substantially violet and the color of the larger particles being bronze.

With respect to the by-products, it is within the purview of the invention to recycle the gaseous by-products' which are formed, in accordance with the following equations:

(1) TiC14+2H2S:TiS2-{4HC1 (2) TiS2+2NH3='IiN+2I-I2S+H2+ %N the hydrogen sulfide produced as a by-product in the secondreaction being recirculated to react with additional titanium tetrachloride to produce additional titanium disulfide which in turn may be reacted with ammonia gas supplemented by the hydrogen and nitrogen by-prod'ucts produced in the secondreaction to form additional-titanium nitride.

In order to more fully describe preferred embodiments of the instant invention, the following examples are given:

Example I In order to prepare the titanium sulfide for re action with anhydrous ammonia, titanium sulfide powder produced by reaction of'titanium tetrachloride with anhydrous hydrogen sulfide was ground to form a powder substantially 90% of which comprised particles of about 40 microns in size. 4.5 grams of the ground titanium sulfide was introduced into the reaction tube of a resistor furnace. After flushing out the'tube with argon to remove any oxygen, the temperature of the furnace was raised to about 600 C. whereupon anhydrous ammonia was introduced into the reaction tube at the rate of substantially 0.056 mol per hour to react with the titanium sulfide, the ratio of the mol parts of ammonia to titanium sulfide being about 4.2 to 1.0.

The reaction tube was rotated slowly and held at the temperature of about 900 C. for three hours at the end of which time the titanium sulfide powder was converted to substantially pure finely divided titanium nitride, the gaseous byproducts produced during the reaction being carried oif after passing through a suitable water trap. After completion of the reaction, the reaction tube was cooled and the titanium nitride removed therefrom. The recovery of the titanium values was substantially 100% and the titanium nitride was a substantially violet colored powder of uniform primary particle size ranging from 0.01 to 0.20 micron and quite free of graphite or free carbon.

Example II Using substantially the same procedure described in Example I, the temperature of the titanium sulfide powder was held at about 1400" C. for about three hours during the reaction. Substantially identical results were obtained as described in Example I with the exception that the primary particle size of the finely divided titanium nitride product was about 0.03 to 0.5 micron, the color of the product being bronze.

The titanium nitride produced by the improved process of this invention is a substantially carbon free finely divided product, the nitrogen and titanium content being as high as 21.6 parts nitrogen and '75 parts titanium as compared to a theoretical nitrogen content of 22.6 parts and a titanium content of 77.4 parts. Because of the, absence of free carbon and the fineness of the particles of titanium nitride formed by the process of this invention, it can be used successfully in powder metallurgy processes to produce moulded articles of great strength and hardness. Moreover, the process itself is a substantially oxygen free system wherein the by-products are of a gaseous nature and are readily removable therefrom, as-a consequence of which the process is characterized by its simplicity, economy and adaptation to "commercial uses.

While this invention has been described. and illustrated by the examples shown, it is not intended to be limited thereto, and other modifications may be employed within the scope of the following claims.

I claim:

1. Method for the preparation of finely divided titanium nitride which comprises providing an anhydrous titanium sulfide powder having a primary particle size of less than 300 microns, and reacting said sulfide powder with anhydrous ammonia gas at a temperature of from about 800 C. to about 1400 C.

2. Method according to claim 1 in which the anhydrous ammonia gas is in excess of the stoichiometric amount for reacting with the titanium sulfide powder.

3. Method according to claim 1 in which the ammonia gas and titanium sulfide are in the ratio of substantially 4 mols ammonia to substantially 1 mol titanium sulfide.

4. Method for the preparation of finely divided titanium nitride which comprises providing an anhydrous titanium sulfide powder having a primary particle size of less than 300 microns; heating said sulfide powder in an oxygen free atmosphere to a temperature of from about 800C. to about 1400 C. and introducing anhydrous ammonia gas into said oxygen free atmosphere at the ratio of about 3.0 mols for each mol of titanium sulfide present, for about 3 to 5 hours.

JOSEPH J. CACCIOITI.

No references cited. 

1. METHOD FOR THE PREPARATION OF FINELY DIVIDED TITANIUM NITRIDE WHICH COMPRISES PROVIDING AN ANHYDROUS TITANIUM SULFIDE POWDER HAVING A PRIMARY PARTICLE SIZE OF LESS THAN 300 MICRONS, AND REACTING SAID SULFIDE POWDER WITH ANHYDROUS AMMONIA GAS TO A TEMPERATURE OF FROM ABOUT 800* C. TO ABOUT 1400* C. 